Distributed hydraulic systems are presently used on most commercial and military aircraft. In a distributed hydraulic system, power is transferred from a power generation source to a power utilization device by hydraulic fluid. For example, the hydraulic fluid for powering the flight control actuators, and other such power utilization devices located throughout the entire aircraft, is distributed under high pressure in hydraulic fluid transmission lines from a pump. The hydraulic fluid must generally be under high pressure, 3000 psi, 8000 psi, or the like, to provide sufficient force for driving the hydraulic actuators. The pump is coupled to and driven by an engine shaft. High-pressure lines extend from the pump to the actuators. Flight control effectors are operated and controlled using these actuators.
Distributed hydraulic systems have numerous disadvantages. They are relatively inefficient because of the large pressure losses occurring in the transmission lines and valves. A break or pressure leak at any single point in the high-pressure line causes loss of hydraulic pressure throughout the aircraft, possibly resulting in loss of control of the aircraft. Significant expense is added to the aircraft because the transmission lines must be leakproof under very high pressure, 3000 psi, 8000 psi, or the like, depending upon the design. In some aircraft, the high-pressure transmission lines are extremely long because of the long distance to the aircraft extremities, such as to the tip of each wing and to the tail, further adding to the weight and likelihood of breakage.
A power-by-wire actuating system for aircraft control systems has been proposed in the prior art. In a power-by-wire system, power is distributed electrically from the power source to the power utilization device rather than hydraulically. The motor-driven pump for providing the high-pressure fluid to the actuator is preferably located as close to the actuator as possible. The assembly of the actuator, pump, and motor is known in the prior art as an "integrated actuator package" ("IAP") or, alternatively, as an "electro-hydrostatic actuator" ("EHA"), jointly referred to herein as an "IAP."
Numerous benefits are provided by using a power-by-wire actuating system. For military aircraft, a significant advantage is enhanced survivability of the aircraft. There are no high-pressure hydraulic lines running throughout the aircraft to the individual actuators, and therefore the likelihood of damage to a hydraulic transmission line is significantly reduced. If one IAP is destroyed with its dedicated hydraulic circuit, the other IAPs continue to operate, permitting the pilot to maintain complete control of the aircraft. Studies have shown significant weight and cost savings are realized with electric wires replacing high-pressure hydraulic lines. Numerous other advantages are provided in a power-by-wire system, including improved performance, reliability, safety, maintainability, power utilization, and the like.
Despite the advantages of a power-by-wire system, none are presently in use in any of today's high-performance military aircraft (although IAPs have been used in the past on British military and commercial aircraft). IAPs operating some flight control surfaces may reach very high temperatures or very low temperatures in flight on a high-performance aircraft. Operation of a hydraulic actuator can cease or degrade if the fluid, or electronics, exceeds a maximum temperature or falls below a minimum temperature. Furthermore, IAP systems, as presently used, require a fluid reservoir for each IAP. Because catastrophic failure results if the hydraulic fluid volume diminishes excessively, a relatively large fluid reservoir must be provided for each IAP.